An anesthesia device which monitors the amount of anesthetic held for vaporization and the amount of anesthetic and carbon dioxide in the mixture of gases in the circular re-breathing system.
During veterinary or human surgical procedures, a conventional anesthesia device entrains an amount of anesthetic into a mixture of gases utilizing an anesthetic vaporizer. The anesthetic entrained in the mixture of gases can be delivered for inhalation by a patient in a circular re-breathing system. The depth of general anesthesia depends on the partial pressure (or gas fraction) exerted by the inhalation anesthetic (or inhalation agent) on the patient's brain. This brain partial pressure of the inhalation anesthetic depends on arterial blood partial pressure of the inhalation anesthetic which depends on the alveolar partial pressure of the inhalation agent which in turn depends on the partial pressure of the inhalation anesthetic in the inhaled mixture of gases. To change the partial pressure exerted by the inhalation anesthetic on the patient's brain, the partial pressure of the anesthetic or inhalation agent is adjusted in the inhaled mixture of gases. The partial pressure of the inhalation anesthetic is equal the mole fraction of the inhalation anesthetic times the total pressure of the inhalation gases.
An important concept in comparing inhalational anesthetics is their measure of potency called the minimum alveolar concentration (MAC). It is defined as the concentration of a particular inhalational anesthetic at one atmosphere pressure in which 50 percent of patients do not move in response to a skin incision. Therefore, the potencies (as well as side effects at similar potencies) of different inhalational anesthetics can be compared; so can combinations of inhalation anesthetics. In general, a half MAC of each of two inhalational anesthetics is equivalent to one MAC of either. The MAC of inhaled anesthetics in one hundred percent oxygen, varies:
Halothane0.74 percentEnflurane1.68 percentIsoflurane1.15 percentDesflurane6.30 percentSevflurane2.00 percent
Conventional veterinary inhalation anesthesia utilizes an anesthesia device that incorporates an oxygen flow meter, an anesthetic vaporizer, and circular re-breathing system with carbon dioxide absorption. As but one example, Isoflurane inhalation utilizing conventional anesthesia devices can provide general anesthesia for a variety of animal species, including without limitation, dogs, cats, birds, mice, rats, guinea pigs, and macaques.
Often a fast acting but short lived sedative is administered to the animal by injection and an endotracheal tube is placed in the animal's windpipe through which oxygen and the inhalation anesthetic are delivered. For isoflurane anesthetic induction, the oxygen flow rate is typically calculated at 100 milliliters per kilogram of body weight per minute and the anesthetic vaporizer set at between three percent to about four percent. Isoflurane anesthetic maintenance is typically achieved by delivery of a minimum oxygen flow rate of about 500 milliliters per minute for animals of less than 50 pounds body weight and an additional 100 milliters per minute for each additional ten pounds of body weight up to 100 pounds of body weight with the vaporizer set to deliver between 1.5 percent and 2.0 percent Isofurane.
A significant problem with conventional anesthetic delivery devices and procedures can be that the anesthetic vaporizer may not be properly readjusted after delivery of anesthetic during the induction period. With regard to both humans and animals, patients have been inadvertently overdosed during anesthetic induction and during subsequent maintenance anesthesia. See for example, Patermann, B., Buzello, S., Dück, M., Paul, M. and Kampe, S., “Accidental Tenfold Overdose of Propofol in a Six-month Old Infant Undergoing Elective Craniosynostosis Repair” Anaesthesia 59 (9), 912-914 (2004). Even during resuscitation attempts oxygen delivered from the anesthesia machine can be contaminated with inhalation anesthetic because the vaporizer has been accidentally been left on the full ON position. See for example, Randall, B., and Corbett, B., “Fatal Halothane Poisoning During Anesthesia with Other Agents” Journal of Forensic Sciences, Vol. 27, Issue 1, (January 1982). With respect to animals, with Irish Wolfhounds and Rottwieler breeds mainly in mind, cases have occurred where dogs have died under maintenance anaesthesia and it is considered that these breeds are sensitive and may not require the dosage its weight might indicate, as such induction and maintenance anesthesia may require more reliable monitoring of general anesthesia.
Another significant problem with conventional anesthesia delivery devices and procedures can be that visual observation of the amount of anesthetic in the anesthetic vaporizer can be required to ensure that the proper amount of anesthetic is held for delivery by the anesthetic vaporizer to the inhalation circuit. By periodic observation and comparison of the amount of anesthetic remaining in the anesthetic vaporizer to a mark inscribed on a viewing aperture an estimate the amount of anesthetic delivered can be made for certain models of anesthetic vaporizers.
However, visualization to estimate the amount of anesthetic in the anesthetic vaporizer and comparison to the prior estimated amount of anesthetic in anesthetic vaporizer to determine the amount of anesthetic delivered from the anesthetic vaporizer may not yield consistent delivery of anesthetic from the anesthetic vaporizer or the proper partial pressure of the inhalant anesthetic delivered to the patient. Inconsistent, inaccurate, or undesired delivery rates of anesthetic from the anesthetic vaporizer or partial pressures of anesthetic inhalants delivered to the patient can result from the failure of or inconsistency of the operator. In certain instances, the operator may simply become distracted from visualizing, or forget to visualize, the amount of anesthetic in the anesthetic vaporizer. Alternately, anesthetic visualization may occur less frequently than required, or the elapse of time between visualization events may vary to a greater degree than necessary to generate a required, predetermined, consistent, or desired delivery of anesthetic from the anesthetic vaporizer or partial pressure of anesthetic inhalant delivered to the patient. Also, visualization by the operator may simply be in error as to the actual amount of anesthetic in the anesthetic vaporizer.
In addition, anesthetic visualization and calibration of the anesthetic vaporizer can be complicated by the numerous different anesthetics which may be delivered to patients, each of which may have unique anesthetic characteristics (density, boiling point, vaporization rate, or the like). As such, vaporizers may be configured differently or calibrated differently for the delivery of each of the various anesthetics.
In other instances, conventional vaporizers may be poorly designed contributing to operator error, fail to operate, operate out of calibration, operate inconsistently, or operate in an other undesired manner, making estimation of anesthetic delivery less consistent, less precise, or in some cases not possible at all. See for example, Buettner, A. U., “Failure of Vaporizer Interlock Mechanism.” Anaesthesia & Intensive Care. 2000; 28:451-2.
Another significant problem with conventional anesthesia delivery devices and methods can be that only visualization or dependence upon calibration of the anesthetic vaporizer may used to estimate the amount of anesthetic delivered from the anesthetic vaporizer to the re-breathing system. As discussed above, reliance on calibration or visualization to estimate the partial pressure of anesthetic in the re-breathing system may not provide information as to the actual condition of the gas mixture inhaled by the patient, or the subsequent condition of the gas mixture exhaled by the patient.
Upon exhalation by the patient the remaining amount of anesthetic gas and the exhaled mixture of gases can be transferred to an absorber to remove carbon dioxide gas (CO2) with a CO2 absorbent. The absorbent, which initially absorbs substantially all the CO2, gradually becomes saturated until the absorbent no longer retains CO2 and levels of CO2 in the re-breathing system can rise to levels harmful to the patient. The absorbent typically contains an indicator which changes color prior to saturation with CO2, however, the saturation point at which the indicator changes color can vary. As such, color change can be unreliable and harmful CO2 levels can build up in the re-breathing circuit of conventional anesthesia device of which the operator can be unaware resulting in harm to the patient.
With respect to the above-mentioned problems associated with conventional anesthesia devices, methods of anesthesia delivery and monitoring of inhalation anesthesia, the present invention addresses each.